As part of a broader interest in complement interactions with blood cells in man, this project focuses on activation of human platelets by immune complexes. Emphasis will be given to an in-depth study of the platelet-stimulating effects of particulate complexes formed by the interaction of zymosan with several types of plasma proteins (complement components, IgG and fibrinogen), as a model system for examining the range of human platelet responses to immunologic stimuli. Particular interest will be focused on: (a) interactions between particle-bound late-acting complement components and the platelet membrane; and (b) the qualitative and quantitative relationships between particle-bound IgG and complement components in promoting binding of complexes to platelets and stimulation of aggregation/release reactions. More selective parallel studies will be made with certain other types of platelet-triggering immune complexes.